This invention relates to a deployment system for an upper bundle cleaning/inspection device for a nuclear power plant steam generator.
Steam generators convert heat from the primary side of a nuclear power plant to steam on the secondary side so that the primary and secondary systems are kept separate. A typical generator is a vertical cylinder consisting of a large number of U-shaped tubes which extend upward from the floor or xe2x80x9ctube sheetxe2x80x9d of the generator. Fluid at high temperature and pressure from the reactor travels through the tubes giving up energy to a feed water blanket surrounding the tubes in the generator creating steam and ultimately power when the stream is introduced to turbines.
Steam generators were designed to last upwards of forty years but in practice such reliability figures have proven not to be the case. The problem is that sludge from particulate impurities suspended in the feed water forms on the tubes which greatly affects the efficiency of the generator and can even cause the tubes to degrade to the point of causing fissures in the tubes. If radioactive primary fluid within the tubes seeps into the secondary side, the result can be disastrous. Plugging or otherwise servicing such fissures is time consuming and results in expensive down time during which power must be purchased from other sources at great expense.
There are known methods for cleaning the tubes proximate the bottom of the steam generator using flexible lances and the like using water under pressure, but since a typical steam generator can be thirty feet tall, it is very difficult to reach the sludge at the upper levels of the tubes using water jets directed upwards from the bottom of the generator.
So, chemical cleaning is used but there are several disadvantages. First, chemical cleaning is very expensive and requires an extended outage. Also, the solvents used in chemical cleaning can corrode the internal components of the steam generator. In addition, chemical cleaning can generate large quantities of hazardous, possibly radioactive waste. Disposal of this waste is very expensive. For these reasons, although many utilities have considered chemical cleaning, few plants have actually implemented it.
On the other hand, there are severe technical challenges faced when considering alternate cleaning methods. A typical steam generator has approximately 50,000 square feet of heat transfer area. The tube bundle is about 10 feet in diameter and 30 feet tall but the access alley in the middle of the tube bundle is only 3.5 inches wide and is interrupted by a series of successive support plates approximately every 4 feet. There are flow slots through the support plates but they are very small in size, typically 2.75 by 15 inches. In addition, the access into the steam generator is limited to a six inch hand hole. Finally, the gap between the vertically extending tubes is only 0.406 or less.
Therefore, manipulating cleaning spray heads and/or inspection equipment up 30 feet to the top of the steam generator to clean or inspect the upper tube bundles is not trivial. There are three primary design considerations. First, the deployment system must be small enough so it can fit through the hand hole of the steam generator and through the flow slots in successive support plates. Second, the deployment system must extend up through the flow slots to a length of as much as 30 feet while still providing support for the cleaning head or inspection device deployed at the distal end. Third, the deployment system must be fully retractable. A system which has the potential for failure, which could become lodged in the upper regions of the tube bundles, or which could fail and leave components inside the steam generator is too risky to employ inside the very expensive steam generators.
In addition, the deployment system must facilitate fast cleaning and/or inspection to minimize downtime with a minimum of manual labor due to the expensive outage costs associated with nuclear power plants and the potential hazard of radioactive exposure to workers in the area during cleaning.
In the patent to Brooks (U.S. Pat. No. 5,265,129), a dual boom design is discussed wherein a telescoping portion consisting of a plurality of pneumatic or hydraulic cylinders is used to deploy an inspection camera up through the support plates. One problem with this design is that the telescoping portion in its collapsed state must be received through the hand hole of the generator and then uprightedxe2x80x94but it cannot exceed the height of the first tube support plate which may be as small as 18 inches.
Such a device which in its collapsed state can be uprighted in an 18 inch height and which is still capable of extending up to 30 feet is difficult to design, manufacture, and control.
It is therefore an object of this invention to provide a deployment system for an upper bundle steam generator cleaning/inspection device which does not rely on telescoping cylinders.
It is a further object of this invention to provide such a deployment system which allows cleaning of the steam generator to proceed from the top down thereby flushing deposits downward during the cleaning process.
It is a further object of this invention to provide such a deployment system which eliminates the need to use chemical cleaning techniques and overcomes the disadvantages inherent in chemical cleaning or which can be used in conjunction with chemical cleaning.
It is a further object of this invention to provide such a deployment system which fits through an access in the bottom of the steam generator, which can be protracted to extend up through the flow slots in the support plates of the interior of the steam generator to deliver a cleaning head or inspection camera to the upper bundles of the steam generator, and which then retracts back down through the flow slots for removal after the cleaning and/or inspection operation is completed.
This invention results from the realization that instead of inserting a device into the steam generator through the hand hole and then relying on telescoping cylinders to deploy a cleaning head or inspection camera up through the tube support plates, a suitable deployment system can instead be constructed by using a snake-like device which is fed through the hand hole from the outside of the steam generator, is flexible enough to make the 90xc2x0 turn to be in position to travel upwards through the tube support plates, and is also rigid enough to then travel upwards to the upper tube bundles of the steam generator (e.g., 30 feet) and still support inspection devices or cleaning heads for inspection or cleaning the upper areas of the steam generator, and which is also retractable so that the inspection device or cleaning heads are safely removed from within the steam generator.
This invention features a deployment system for an upper bundle steam generator cleaning/inspection device. The deployment system may suitably comprise, include, consist essentially of, or consist of an elongated body feedable through an access in a steam generator shell proximate the tube sheet of the steam generator. The elongated body is flexible in one configuration to bend into a position for extension up through flow slots in support plates of the interior of the steam generator and rigid in another configuration for positioning and supporting cleaning/inspection devices up through the steam generator proximate the upper tube bundles of the steam generator. There are also some means for driving the elongated body up through the support plates and for retracting the elongated body back down through the support plates.
The elongated body assembly typically includes means for mounting a cleaning head on a terminal end thereof for cleaning the upper tube bundles of the steam generator and/or means for mounting an inspection camera on a terminal end thereof for inspecting the upper tube bundles of the steam generator.
The elongated body may be a rigid chain, or a pair of rigid chains each bendable in only one direction, each deployed into the steam generator by bending, the pair deployed back to back in the rigid configuration.
Another type of rigid chain includes a number of links, each pivotable with respect to an adjacent link in one configuration, the links including means for releasably locking adjacent links against pivoting in another configuration. The means for releasably locking includes retractable pins for locking the links together when engaged, and for freeing said links when retracted. In this case, the means for driving includes means for automatically retracting and engaging the pins.
The means for releasably locking may alternatively include detent balls on one portion of the links and complementary detent recesses on one portion of adjacent sets of links or, the means for releasably locking may include a spring for urging one link to remain engaged with an adjacent link. The means for releasably locking may be a magnet for urging one link to remain engaged with an adjacent link. Also, the means for releasably locking may be both a spring and a magnet for urging one link to remain engaged with an adjacent link. The rigid chain could also be a plurality of links each having a hinge and a portion extending beyond the hinge for preventing movement of an adjacent link in one direction.
As an alternative to the rigid chain configuration, the elongated body may include a plurality of rigid links. The links each have a hinge and at least one articulation recess proximate the hinge for allowing movement of an adjacent link in only one direction, or there may be an articulation recess on each side of the hinge. The deployment system may also be an extendable mast formed of a material self-biased to form a tube. In this case, the means for driving includes a pair of counter-rotating drums for driving the mast material engaged between the drums.
The mast configuration and the rigid chain configuration or the rigid link configuration may be combined: the elongated body comprises a rigid chain supported by an extendable mast formed of a material self-biased to form a tube or the elongated body comprises a series of rigid links supported by a mast formed of a material self-biased to form a tube.
The drive means preferably includes a turning shoe for directing the elongated body from a position proximate the tube sheet to a position for extension upwards therefrom to the upper bundles of the steam generator.